Recent years have seen an increasing demand for latent heat storage devices, particularly in vehicular applications where they are employed as latent heat batteries. A typical latent heat battery typically includes one or more containers of a phase change material (PCM). The PCM absorbs and releases energy in the form of sensible and latent heats, as the material is heated or cooled and/or changes phases between liquid and solid in the heating or cooling process. A typical source of heat is the coolant from a vehicle engine which is flowed about the container of phase change material. When the latent heat battery is charged, the phase change material will be in the liquid phase. Consequently, if cold coolant is circulated about the container of phase change material, the coolant will be warmed as the phase change material is cooled and solidifies, giving up sensible heat and the latent heat of fusion. The now warmed coolant may be utilized to provide relatively instant heat to the interior of the vehicle if passed through the vehicle cab heating system. It may also be utilized to warm the engine and/or transmission to provide for a much faster warm-up than would occur simply through operation of the engine alone.
The latent heat battery, of course, becomes discharged as it warms the coolant and must be recharged. This is accomplished simply by running the coolant through the latent heat battery while the engine is operating a t normal operating temperature. The now hot coolant will warm the phase change material causing it to return to the liquid state from the solid state, absorbing sensible heat and the latent heat of fusion.
In the usual case, one or more containers of a phase change material are disposed within a housing, conventionally referred to as the salt jacket or the inner jacket or the cooling jacket. The salt jacket, in turn, is located in spaced relation, within an outer jacket. Insulating material may be disposed between the interior of the outer jacket and the exterior of the salt jacket. Additionally, a vacuum may be pulled between the two jackets to provide good insulation. The insulating material that may be located in the space between the salt and outer jackets is intended to block heat transfer from the salt jacket to the outer jacket by radiation or to reduce convection when a lesser vacuum is used or as the vacuum deteriorates. Through the use of this insulating technique, the charge on a latent heat battery may be maintained for several days.
While latent heat batteries are not limited to vehicular uses, in vehicular uses, they may be located in any convenient location near the engine compartment. In a current application, the latent heat batteries are located in a recess in the floorboard in the passenger compartment on the floor of the passenger side of the vehicle. Consequently, there is a limited amount of space available for the heat battery. Furthermore, in the general considerations employed in vehicle design, it is highly desirable to make any component as lightweight as possible so as to increase fuel efficiency. Thus, it is highly desirable to make the latent heat battery as small and as lightweight as possible. Minimal size and weight can be achieved by using a PCM with a high total heat capacity (i.e., sensible and latent heat capacity). Generally, merely reducing the size of the latent heat battery is not a sufficient response to the problem. Reducing the size of the battery also reduces its capacity to store heat in that, as the volume of the latent heat battery is reduced, the quantity of PCM that it can accommodate is concomitantly reduced, thereby reducing its heat capacity. Consequently, minimizing weight of the battery will generally be achieved through the use of lightweight materials, such as aluminum.
It is also necessary that salt and outer jackets of the latent heat batteries be structurally reinforced. This is particularly important when a vacuum is pulled in the space between the outer jacket and the salt jacket. Furthermore, since the coolant flowing through the salt jacket is under pressure, a large pressure differential between the inside and outside of the salt jacket is created, causing the salt jacket to tend to deflect outward. Similarly, the outer jacket has a nominal pressure on the outside of one atmosphere and vacuum on the inside, causing the outer jacket to tend to deflect inward. As the salt and outer jackets collapse toward one another, the space between the jackets is reduced. This reduces the insulating ability of the insulating space. Further, as the salt jacket deforms, it in turn may deform the PCM container(s) contained in the salt jacket. Furthermore, every time the vehicle is operated the coolant pressure oscillates between one and two atmospheres as the vehicle heats up. This results in a fatigue cycle being imposed on the PCM container(s). Similarly, the salt and outer jackets must be sufficiently rigid that the jackets do not collapse toward one another reducing the insulating space, thereby reducing the insulating ability of the insulating space, with a consequence that the battery cannot retain a charge as long as might be desired.
Finally, many phase change materials in use today undergo significant volumetric changes in the process of changing from the solid phase to the liquid phase and back. Again, the phase change material containers must have sufficient strength to avoid rupture in response to such volumetric changes.
All of these factors have limited the success of latent heat batteries in vehicular applications, either by posing extreme limits on the latent heat capacity or on the charge that may be placed on the latent heat battery.
In other applications, different problems exist. For example, many types of apparatus require some means of heat rejection to prevent equipment from overheating. One example is in apparatus employing electronics. Heat generated by semi-conductors or chips during their operation must be dissipated to prevent their destruction as a result of overheating. To accomplish this, cooling systems are employed whereby a coolant is flowed in heat exchange relation with the apparatus components requiring cooling. In many types of apparatus the heat load is not uniform. During the course of operation of such equipment, large heat spikes may be generated and the cooling system must be designed to accommodate the heat spikes and reject the heat represented by them.
Heretofore, that has required the use of oversized cooling systems which have sufficient capacity to reject the necessary amount of heat for the highest heat generation encountered during operation of the equipment, i.e., has a cooling capacity to reject the maximum amount of heat that is present as a result of heat spikes. To accomplish this goal, many of these cooling systems are unnecessarily large in order to have the desired heat rejection capacity. This contributes to problems with the sheer size of the cooling system, the use of additional material in forming the components, additional energy costs in operating the cooling system due to the need for large fans and pumps, etc.
The present invention is directed to overcoming one or more of the foregoing difficulties so as to provide a light weight, high capacity, low volume latent heat storage device.